Two days ago, I met a customer, and when I came up, I asked: Is your rubber metal expansion joint just an iron shell on the outside of the rubber compensator? As soon as I heard it, I knew that this brother had probably stepped on a trap before. Actually, this thing looks like rubber and metal together, but it's really not that simple.
1. Rubber-metal expansion joint is not as simple as "rubber + metal"
First, let me know the difference between it and rubber compensator and metal expansion joint.
The main body of rubber compensator (also called rubber joint) is pure rubber plus ply, and the elasticity of rubber and ply strength are used to compensate the displacement. Its shortcomings are obvious-the pressure resistance is not high. As soon as the temperature exceeds 80℃, the rubber begins to age and become sticky, and it has to be replaced after two years in the hot water system.
Metal expansion joints (such as the general corrugated expansion joints and high-temperature axial expansion joints in our station) are made of stainless steel bellows, which can withstand high temperature and high pressure, but they have poor absorption capacity for mechanical vibration and lateral displacement, and are expensive and heavy.
What about rubber metal expansion joints? It is a structure that combines rubber elastomer with metal flange or reinforcing ring. The rubber part is responsible for vibration absorption and compensation for angular displacement and lateral displacement; The metal section provides structural strength, pressure bearing capacity, and connection rigidity. To put it bluntly, it is a "hybrid"-both the flexibility of a rubber compensator and the ability to withstand much higher pressure than pure rubber parts. Common structures are: inner layer of corrosion-resistant rubber + multi-layer cord reinforcement + outer layer of metal protective ring or flange. For example, the rubber compensator series in our station can be upgraded to a rubber metal expansion joint if a metal reinforcement structure is added, but the two are not the same thing.
2. What working conditions must be carried out? Which working conditions use it to cause trouble for yourself?
Scenario that must be used:
- A piping system having a medium temperature between-10°C and 120°C and a pressure between 1.0 MPa and 2.5 MPa. For example, circulating water system, aeration pipeline of sewage treatment plant, fire pipeline.
- Pump room with strong vibration, compressor outlet pipeline. The rubber metal expansion joint has first-class vibration absorption ability, which can obviously reduce the probability of pipeline fatigue cracking.
- Pipe sections that need to be disassembled and cleaned frequently-because of the flange connection, it is easy to disassemble and assemble, unlike metal expansion joints that can't move when they are welded to death.
Never use scenarios:
- Steam pipes with media temperatures exceeding 150°C. The rubber layer will be carbonized and failed. At this time, high-temperature axial expansion joint or external pressure single axial expansion joint should be selected.
- The medium is a strongly oxidizing acid (e.g. concentrated nitric acid, concentrated sulfuric acid) or an organic solvent (benzene, ketone). The rubber will be swollen or corroded, and it is more reliable to replace it with a PTFE-lined hose or PTFE compensator.
- Negative pressure system with vacuum degree above 0.08MPa. The rubber layer of the rubber metal expansion joint is easy to delaminate and deflate under negative pressure, so it is necessary to use a vacuum special hose with an inner support ring.
And guess what? Last year, a power plant customer used rubber metal expansion joints on desulfurization flue gas pipelines. The medium contained gypsum slurry, pH =2, and the temperature was 70℃. As a result, the sealing surface of the flange was corroded to pieces in three months. Later, it was replaced with a rubber PTFE compensator (we have this product in our station), and the PTFE layer was lined to hold it. Selection rollover, doubling the cost.
3. Select the three parameters that are most prone to rollover: pressure, temperature and media corrosiveness
Don't just look at the maximum value of the superscript on the sample.
Pressure:The nominal pressure (PN) of rubber metal expansion joints is usually marked on the flange, but the actual allowable working pressure is linked to the medium temperature-for every 10℃ increase in temperature, the elastic modulus of the rubber drops by a section. For example, a certain product can withstand a pressure of 1.6MPa at room temperature, but may only remain 1.0MPa at 100℃. Therefore, "pressure-temperature curve" must be asked for when selecting, and products without curve charts will be treated as false labels. We have actually measured a domestic sample, nominal PN1.6, which bulged after only 5 minutes under the hydraulic test at 110℃.
Temperature:The temperature resistance limit of rubber depends on the rubber type. The temperature resistance range of ethylene propylene diene monomer (EPDM) is-40℃ ~120℃, nitrile butadiene rubber (NBR) is oil resistant but not high temperature resistant, and neoprene rubber (CR) has good weather resistance but the upper temperature limit is only 80℃. So ask what kind of rubber the supplier uses, and don't believe the vague statement of "temperature resistance 150℃"-unless it is fluororubber (FKM), but the vulcanization of fluororubber is difficult and costly, and small factories can't do it well.
Media corrosivity:The inner rubber layer of the rubber metal expansion joint is in direct contact with the medium, while the metal flange and reinforcing ring are exposed. If the medium contains chloride ions (e.g. seawater, some industrial wastewater), the metal part should be treated with 316L stainless steel or PTFE lined. It is recommended to ask the supplier directly for the Chemical Compatibility Chart, which lists how corrosive several hundred media can be to different rubbers and metals. Don't have this table? Direct pass.
IV. Installation and life: Should the tie rod be disassembled? How to adjust the screw?
This is the most asked question by customers, and it is clear today.
Do you want to remove the tie rod?
When rubber metal expansion joints leave the factory, there are usually temporary tie rods and nuts on both sides of the flange to fix the installation length and prevent twisting and deformation during transportation. Once installed in place,Tie rod must be removed！ If it is not disassembled, the expansion joint cannot be freely expanded and contracted to compensate for the displacement, which is equivalent to installing a dead piece. The thermal expansion and contraction stress of the pipeline is all suppressed on the flange weld, which is easy to crack.
Then when won't you dismantle it? When the pipeline has an axial thrust limit, the limit rod (also called the limit bolt) can be retained, but the nut position needs to be adjusted according to the design displacement. For specific adjustment methods, refer to "Does the screw of the expansion joint need to be disassembled" and "How to adjust the tie rod nut of the expansion joint" often asked in our station-simply put: loosen the locking nut first, so that the bolt can slide freely; After installing the expansion joint, adjust the gap between the end of the bolt and the limit plate (generally 1/2 of the designed compensation amount), and finally tighten the locking nut. Don't screw too hard, leave room for activity.
There are two more details when installing:
- Medium-resistant sealing gaskets should be added to the flange connection, and asbestos gaskets should not be used (the expansion of asbestos after absorbing water will deform the rubber flange). Teflon pads or metal wound pads are recommended.
- The expansion joint cannot directly bear the weight of the pipe, and must be provided with an independent support. A common mistake is to use the expansion joint as a hanger, resulting in fatigue tearing of the rubber layer due to long-term stress.
Under normal working conditions (temperature ≤100℃, pressure ≤1.0MPa, no strong corrosion in the medium), the design life of rubber metal expansion joint is about 8~10 years. However, if you find that the flange sealing surface leaks, and the rubber surface is cracked or bulged, you must replace it immediately. Don't wait until the tube bursts.
5. How to choose the same type of products? Rubber compensator, rubber PTFE compensator, non-metallic expansion joint-where are their respective home fields?
Many customers come to find a quotation with the word "expansion joint". In fact, there are many doors inside.
Rubber Compensator:Suitable for low pressure (≤1.0MPa), low temperature (≤80℃) water or air medium, low cost, flexible installation. However, the corrosion resistance is average, and it is not suitable for oils and solvents. This is how the rubber compensator series in our station is positioned.
Rubber PTFE Compensator:Lined with polytetrafluoroethylene (PTFE) film, it is resistant to almost all chemical media (except high-temperature molten alkali and elemental fluorine), and has a low friction coefficient, which is not easy to scale. Suitable for acid, alkali and salt solution pipeline of chemical plant, the temperature can reach 180 ℃. The disadvantage is that the price is 2~3 times more expensive than the ordinary rubber compensator, and it can't bear negative pressure.
Non-metallic expansion joints (fabric fiber expansion joints):Made of composite materials such as silica gel cloth, fluorine adhesive cloth and glass fiber cloth, the temperature resistance can reach 400℃ or even higher, and the weight is light and the compensation amount is large, and it is suitable for large-section rectangular pipes such as smoke duct and dust collector inlet and outlet of power plant. However, the withstand pressure is extremely low (usually within 0.1MPa), so it cannot be used for liquid pressure pipes. This is the way the rectangular non-metallic expansion joint in our station works.
If there is water, pressure and vibration, choose rubber metal expansion joint; If there is corrosion, temperature and chemical medium, select rubber PTFE compensator; If there is high temperature, dust and large displacement, choose non-metallic expansion joints.